1. Field of the Invention
This invention relates to a process for producing alkyl tert-alkyl ethers useful, among other things, as octane improvers in gasoline compositions.
2. Description of the Prior Art
It is known that alkyl tert-alkyl ethers can be prepared by reacting a primary alcohol with an olefin having a double bond on a tertiary carbon atom, thus methanol reacts with isobutylene and isopentenes (2 methyl 1-butene or 2 methyl 2-butene) to form respectively methyl tert-butyl ether (MTBE) and methyl tert-amyl ether (MTAE).
The reaction is selective for tertiary olefins so that it constitutes a valid process for their removal from olefinic streams in which they are contained together with linear unreactive olefins.
The reaction has an equilibrium which is the more favorable to the synthesis of the ether the lower the reaction temperature in accordance with its negative enthalpy.
It is known that the reaction is catalyzed by Lewis acids (aluminium trichloride, boron trifluoride), mineral acids (sulphuric acid) and organic acids (alkyl and aryl sulphonic acids, ion exchange resins).
Particularly suitable for the task are ion exchange resins in their acid form and it is known that the best results are obtained by means of macroreticular resins of the type "Amberlyst 15".
By means of such last named catalysts it is possible to reach thermodynamic equilibrium within industrially acceptable contact times in the temperature range of 50.degree.-60.degree. C.
At lower temperatures, thermodynamically more favorable, the kinetics are not sufficiently favorable to permit reaching equilibrium in practice.
This fact limits conversions.
Obviously the conversion of a reagent can be increased by increasing in the feed the content of the other reagent but this involves a lowering of the conversion of the excess reagent.
This can cause some drawbacks, as for instance in the synthesis of MTBE starting from methanol and isobutylene contained in an olefinic stream, the use of excess isobutylene involves the fact that the olefinic stream after separation of MTBE still contains 5-10% isobutylene and this constitutes a drawback when such stream has to be utilized for the production of maleic anhydride or butadiene. On the other hand an excess of methanol renders the purification of MTBE very expensive because of the formation of azeotropes.
It is to overcome the foregoing drawbacks in the production of tert-alkyl ethers that this invention is directed by employing a zeolitic catalyst material instead of an ion-exchange resin. Not only are the zeolitic catalysts substantially perfectly stable (which ion-exchange resins are not) but they also suppress formation of diisobutylene during the conversion to tert-alkyl ethers.
Unable to meet the demand for unleaded gasoline, petroleum refiners are turning to other chemicals to extend available supplies and obtain more usable fuel out of a barrel of oil. The chemicals, compounds of carbon, hydrogen and oxygen, contain no metal and avoid the environmental problems associated with tetraethyl lead and MMT (methylcyclopentadienyl manganese tricarbonyl).
Currently much of the interest in these blending agents centers on methyl tert-butyl ether, MTBE, which has been approved for use in motor fuels in concentrations of up to 7 percent by the Environmental Protection Agency.
Since MTBE is derived from crude oil fractions unsuitable by themselves for use in gasoline, simply adding it to the mixture called gasoline adds to gasoline supplies; but MTBE has another useful property; it enhances the octane rating of gasoline.